In many types of network there is often a need for a common frequency reference. A frequency reference may be used directly, or may be used to determine phase and time-of-day. In a communications network, one requirement for the frequency reference is to ensure that service buffers do not over or underflow, especially for long-lived information flows, and certain services also require phase control and time to various degrees of accuracy, which relies on the local availability of an accurate frequency reference.
Atomic references are some of the most stable references available today, but they are prohibitively expensive for many applications. The Global Positioning System (GPS) and other navigational satellite systems can be used for these purposes, but still add significantly to the cost of low-end applications, and the antennas for these satellite signals are expensive to install, not always possible to site, and the signal is easily jammed. Affordable local oscillators can exhibit significant frequency drift in use. There are various known techniques for synchronizing slave oscillators with a master reference oscillator. The physical layer of a network can be used to disseminate frequency synchronization information by extracting a clock signal from the line code protocol used to code data traffic, or by extracting a clock signal from regular data transitions. This technique must be designed into the physical layer protocol of the network from the outset, and is typically used on a per-link basis. To extend this to distribute frequency throughout a communications network also requires that the synchronization systems of all the switching nodes forming the chain between the service entry point (master) and exit point (slave) extract the frequency from a reference link and disseminate it on all output links with a high degree of accuracy. However the challenge with the physical layer method is that as the length of the distribution node and link chain increases, the frequency signal degrades as each successive node is traversed. These impairments are traditionally termed jitter and wander and for performance purposes must be minimised.
While the method is reasonably reliable, and has been used extensively in Synchronous Digital Hierarchy (SDH) networks, this has not been hitherto true of packet network nodes that do not rely on an SDH layer or that do not function as part of a transmission layer. There has been no need to provide an SDH-like synchronization capability because packet networks are asynchronous in operation. In particular, packet network physical links such as Ethernet do not perform to anywhere near similar accuracy as SDH. Furthermore, each Ethernet link operates autonomously from all other Ethernet links on the same node.
The ability to replace SDH networks with packet networks relies on the ability to replace the frequency distribution architecture of SDH. Wireless basestations also require high quality frequency (and often phase) references and need to benefit from the opex and capex savings of packet networks. Applying the physical layer method directly to packet networks would incur a complete redesign of all the nodes and possibly even the physical layer protocols and devices. Unless packet network nodes are upgraded to send frequency synchronization in a native packet physical layer protocol, the only alternative is to use packet transmission directly i.e. use the packet layer above the physical layer for synchronization purposes. One simple method is to use packets to disseminate, on some regular basis, timestamps generated from the master's clock to all slave clocks. Each slave can derive frequency from the timestamps. However, the packet layer is not a constant communications channel, and it is highly susceptible to load variations at the packet switching nodes where varying amounts of traffic from varying numbers of source links desires to exit on one link. This load variation gives rise to generally unpredictable statistics that causes packet arrival on any route between a master and slave to vary in its transfer delay (load induced packet jitter, or just load jitter) as well as potentially suffer lost packets when resources may overflow. These characteristics are dependent both on the amount of load as well as how it varies in time, on the nature of the load in terms of its packet size distribution, its instantaneous and average rates, and its shape, for example its burstiness, and where the load is applied in the network and interplays with other loads.
Even where a load profile may be constant and regular in time, packet size and rate, and within the capacity of all links on the route, packet switching nodes will still cause their own form of delay variation termed switching jitter. This may occur due to internal timing variations that effect the absolute ordering of packets of a timing distribution service relative to other loading services (queuing effects), or because any load services and the timing distribution service have in general inter packet gaps that are not related in a regular way. In general, switching jitter and load induced packet jitter are not correlated, and load jitter is dominant. Furthermore, the amount of load or utilisation relative to capacity in general is slower changing than the delay variation distribution it induces. In general this highly statistical and unpredictable behaviour of the packet layer can severely impair a clock signal, since any variation in delay across the network interferes with the instantaneous frequency rate and the information that can be extracted from the time distribution service packets.
There are several known packet layer techniques for distributing time signals that try to overcome the statistical impairment. A first known technique is a differential technique that measures the source clock of interest relative to a reference that is commonly available to both the master and the slave. The difference between the source clock and reference is transmitted to the slave to reproduce the original frequency relative to the common reference. This is most useful where it is necessary to send an ad hoc frequency between two points, for example a customer's clock relative to a communications network clock. The technique avoids the statistical impairment but suffers from the need to have a common high quality reference, which to date has relied on the communications network clock disseminated at the physical layer in SDH networks. The best example of this technique is ITU-T I.363.1 ATM AAL-1 Synchronous Residual Time Stamp (SRTS) using the SDH clock as reference. Clearly, the technique, albeit using the packet layer, requires a physical layer clock or equivalent reference to operate.
A further known technique is Adaptive Clock Recovery (ACR) which relies on discerning the master's original clock frequency from packet arrival times, and/or timestamps the master has inserted into a packet stream. For simplicity this technique often relies on the master sending packets at a regular interval. This technique is therefore a feature usually supplementary to a constant bit rate communications service that the packet stream represents. Proprietary algorithms are used in commercial implementations of the slave device that invariably rely on Phase Locked Loops (PLLs) and filters to reduce the noise of the packet jitter induced by any network loads. The challenge for this technique is that jitter induced by a load with high transient changes of small amplitude (level) are very difficult to discriminate from the general switching jitter (noise)—a non-stationary distribution, as well as a load that changes only very slowly i.e. over a long time; the former can radically change the delay through the network upsetting the phase of the PLL, and the latter becomes increasingly difficult to filter at ever lower frequencies, which ultimately result in a varying frequency offset in the slave. Examples of this technique are ITU-T I.363.1 ATM AAL-1 ACR, and the Internet Engineering Task Force Circuit Emulation Services over IP (IETF CESoIP) or Structure Agnostic TDM over Packet (SAToP) Real-Time Protocol (RTP) Timestamp clock recovery.
A further known group of techniques are bidirectional phase offset techniques that attempt frequency synchronization simultaneously with phase and time-of-day or Coordinated Universal Time (UTC) synchronization. These techniques typically derive frequency from any phase synchronization, so the quality of phase synchronization determines the quality of frequency synchronization. Examples of this technique are standardised as IEEE1588 and IETF NTPv1 to NTPv4 the latter still under construction. These phase offset techniques use the packet network to send timestamp information in both directions between the master and slave, or rely on measurements of the roundtrip time of the bidirectional route between master and slave. All these techniques have to make the assumption that the delay in both directions of the route is equal, because there is a paradox that it is impossible to discern the delay in each direction separately without first synchronising the phase of the slave clock. The phase of the slave clock at any instant prior to synchronization is certainly arbitrary relative to the master, and is used to timestamp arrival and dispatch of packets at its own end. Similarly a measurement of the roundtrip time cannot apportion delay to each direction without knowledge of arrival time at the slave with reference to the master's clock, so the roundtrip time is simply halved. By assuming the delays of each direction are equal, the phase of the slave is set to be an appropriate correction for half the round trip delay after the master clock. However this still does not make the phase of the slave clock knowable at the master because the delay to the slave receiving this instruction remains unknown. Frequency is derived from phase under all conditions by differentiation; i.e. frequency is equal to the rate of phase change or drift. Corrections for phase offset result directly in frequency offsets therefore. Unfortunately, the delays of a packet network in different directions even for the same route are seldom equal, and for many networks the routes may differ in a physical sense in the directions master to slave and slave to master in terms of number of nodes, links and their distance. Moreover, the delay over the route in either direction can vary considerably and independently as load in each direction varies in the network. Even where steps are taken to detect all these situations, which cannot be mathematically exact but only heuristic, there will be a residual phase offset caused at the slave, or corrections continuously applied with concomitant failure of frequency synchronization. Elaborate algorithms are required to smooth out the packet switching jitter noise and variation of the distribution under load, but even constant loads will still cause constant phase offsets, and high transient loads can cause significant frequency offsets, or force the slave into a holdover mode that incurs its own drift by being decoupled from the master, by definition, at least for the duration of the disturbance.
Accordingly, there is a need for an alternative technique for achieving synchronization between a master and a slave.